when I was in this situation a couple of months ago, I found a thread (via search, sorry don't mean to be rude) where I gentleman had bored his 350f for 400f pistons and IIRC upgraded the cam chain. The mods were detailed under a high mileage thread. I ended up sticking with my 350f pistons and stock rings.
Wait! I remember the formatting of the guy's post was funky, so I saved the post and just reformatted it for easier reading. Here it is:
I posted some history on another thread but can't remember where. Where? where? Oh where was I? Bought the machine new in 1972 for commuting as was tired of my 2 stroke Kwacker smoking & loading up in low speed traffic. The Kwacker was great for canyon thrashing but lousy for low speed commuting. The 350F was nice for its intended work but rather anemic in our mountainous locale.
At about a couple of years old & maybe 25,000 miles or so the engine locked up, in a turn of course. Only minor damage to either of us but upon taking the engine apart I found the primary link belt chain had started to come apart due to defective link pivots & jammed. I was stamped "Hyvo" & Made in USA! the replacement was Japanese & has served since without problems. Since I had the motor apart anyway & oil was leaking from the head joint some experamenting was done.
I researched interchangability with other Honda fours & found that the crankshaft, rods & wristpins are in common with the 400F, as are the camshaft, valves & rocker box. Cases, cylinder. pistons & head are different. I measured 350F cylinder liners & compared with 400F pistons.
If bored to maximum over bore of the 400F pistons the engine would yield 425cc. The original bore was 47mm & the maximum would be 52mm. The large bore make the cyclinders thin at the bottom but I compared with my BSA B50MX, (500cc scrambler), with the 545cc kit & found it to be about the same. The BSA engine gave no problems in open desert scrambles except being difficult to start, at least for me. I turned the outer crowns of the pistons to prevent contact with the stock head. Replaced all the gaskets, etc & several rocker arms due to flaking of the hard surfacing of the pads. It looked like hard chrome facing. Was astonished at the increase in low end torque & geared the bike to reduce the RPM at 70MPH from the original 7,000 to 6,000, about the same as a 400F. Another 25,000 miles or so & a lot of oil leaking from the head again due to the rubber seals around the oil restrictors sqaushing into the overlarge hole in the head gasket. Made up supporting rings surrounding the seals to confine them & they now hold up for many 10's of thousands of miles without leaking. Also more of the rocker arms were damaged & some cam wear evident. Having the head off again gave the opportunity for more enging development.
I replaced the trimmed pistons with unmodified new ones & chamfered the edges of the combustion chambers to match the piston profile, which yielded a perimeter squish band similar to more modern design. While the heads externally had the smooth appearance of die casting the ports obviously were cast using sand cores as they were all rough & irregular. Machining marks in the ports showed that they had been milled a short distance at both ends but nothing touched deeper. Using a porting tool, riffle files, coarse & medium abrasive strips & a lot of hard work I blended the contours & surfaces inside but did not change the shape or size. The improved engine ran better but was restricted with the original exhaust & air filter. E
xperamented with various 4-2 systems but found the 400F pipe & a large unrestricted muffler gave the best all around results. Replaced the stock, restrictive air filter material with Uni-Filter foam cut from bulk stock. Naturally, all this made the carburation lean but upping the main jets cured that but at a cost of about 10MPG. Had to upgrade the clutch to a Barnett 400F racing unit as the 350 could not hold the power. The change is astounding! On a steep grade with a headwind the original motor could not pull top gear. Now with taller gearing it pulls the same grades at part throttle & cruises comfortably at 80MPH although top speed has only increased a few, from 92 to 98 but I have seen 100 on the clock on occasion. The big change is in widening the power band. It will pull full throttle from 25MPH in top with modest acceleration 'til 5,000 when the pull is strong & gets better with the revs up to 11,000. That's a useful power band greater than 2 octaves! No, it can't keep up with my 750 Bonneville but the performance compares well with my G12CSR but is more reliable. Since then I've rebuilt the starter solenoid switch a couple of times & replaced the handlebar controls. Replaced the cam chain twice using the rivet on link from the CB450 twin motor. Replaced cam chain tension blade & guide twice. Recently a top ring shattered taking out the piston & liner too so replaced all that. Replace the oil pressure sender to many times to count but no other major parts & has never left me stranded although my buddies think I'm a maniac. I tell them the truth: I learned to ride in Germany & ride like any European & they consider the pace as fast cruising.
Long & wordy, but we are talking 33 years & almost 200,000 miles here with a lot of experiences.
Cheers, Don Pscycle Madden.
